1. Field of the Invention
The invention relates to polyaromatic ethers, aromatic ether oligomers, phthalonitrile monomers containing aromatic ether oligomer spacers, thermosets made from such phthalonitrile monomers, and processes for making the same.
2. Description of the Prior Art
Phthalonitrile monomers and phthalonitrile polymers of various types are described generally in U.S. Pat. No. 3,730,946, U.S. Pat. No. 3,763,210, U.S. Pat. No. 3,787,475, U.S. Pat. No. 3,869,499, U.S. Pat. No. 3,972,902, U.S. Pat. No. 4,209,458, U.S. Pat. No. 4,223,123, U.S. Pat. No. 4,226,801, U.S. Pat. No. 4,234,712, U.S. Pat. No. 4,238,601, U.S. Pat. No. 4,259,471, U.S. Pat. No. 4,304,896, U.S. Pat. No. 4,307,035, U.S. Pat. No. 4,315,093, U.S. Pat. No. 4,351,776, U.S. Pat. No. 4,408,035, U.S. Pat. No. 4,409,382, U.S. Pat. No. 4,410,676, U.S. Pat. No. 5,003,039, U.S. Pat. No. 5,003,078, U.S. Pat. No. 5,004,801, U.S. Pat. No. 5,132,396, U.S. Pat. No. 5,159,054, U.S. Pat. No. 5,202,414, U.S. Pat. No. 5,208,318, U.S. Pat. No. 5,237,045, U.S. Pat. No. 5,242,755, U.S. Pat. No. 5,247,060, U.S. Pat. No. 5,292,854, U.S. Pat. No. 5,304,625, U.S. Pat. No. 5,350,828, U.S. Pat. No. 5,352,760, U.S. Pat. No. 5,389,441, U.S. Pat. No. 5,464,926, U.S. Pat. No. 5,925,475, U.S. Pat. No. 5,965,268, U.S. Pat. No. 6,001,926, and U.S. Pat. No. 6,297,298, all incorporated herein by reference.
The above references generally teach methods for making and polymerizing phthalonitrile monomers. Such monomers typically have two phthalonitrile groups, one at each end of a connecting spacer chain. The monomers can be cured, where by the cross-linking occurs between cyano groups. These cross-linked networks typically have high thermal and oxidative stability.
Phthalonitrile monomers with aromatic ether oligomeric or polymeric spacers are expected to be useful because they are predicted to have low melting points. Phthalonitrile monomers with a large window between the melting point and the cure temperature are desirable to control the rate of curing and the viscosity during curing.
U.S. Pat. No. 4,259,471 to Keller et al. discloses a phthalonitrile monomer having a polyphenoxy spacer with from 1 to 10 phenyl groups in the spacer chain. The monomer is made by reacting 4-nitrophthalonitrile with an aromatic diol. The aromatic diol is a phenoxy chain with terminal hydroxy groups. The patent states that the aromatic diol can be made by an Ullmann synthesis. However, the patent does not teach how to make the aromatic diol with more than two phenylene groups. It is known in the prior art that an Ullmann synthesis can be used to create a single aromatic ether linkage by reacting a haloaromatic with a hydroxyaromatic in the presence of a stoichiometric amount of a copper complex. There are no known prior reports of the use of an Ullmann synthesis to make an oligomeric or polymeric aromatic ether containing three or more aromatic groups.
U.S. Pat. No. 6,297,298 to Keller et al. recites a phthalonitrile monomer having a polyphenoxy spacer as an embodiment of a general structure. The patent does not disclose any examples of or a process for making this phthalonitrile monomer.
The compound m-bis[m-(m-phenoxyphenoxy)phenoxy]benzene is a commercially available aromatic ether oligomer. There are no other known prior reports of other aromatic ether oligomers
Marcoux et al., J. Am. Chem. Soc. 1997, 119, 10539, discloses a method for synthesizing a diaryl ether from a haloaromatic and a phenol using a catalytic amount of a copper complex and cesium carbonate. This method does not require the harsh conditions of an Ullmann synthesis such as high temperatures. The method also avoids the use of a stoichiometric amount of copper. The publication does not disclose any use of the method to make an aromatic ether oligomer.
There is need for process to make an aromatic ether oligomer and a polyaromatic ether. The resulting aromatic ether oligomer can then be reacted with a nitrophthalonitrile to make a phthalonitrile monomer. The phthalonitrile monomer can then be cured to form a thermoset.
It is an object of the invention to provide a polyaromatic ether and an aromatic ether oligomer.
It is a further object of the invention to provide a phthalonitrile monomer with an aromatic ether oligomer spacer.
It is a further object of the invention to provide a thermoset made by curing a phthalonitrile monomer with an aromatic ether oligomer spacer.
These and other objects of the invention are accomplished by process of preparing a polyaromatic ether comprising the formula: 
wherein Ar is an independently selected divalent aromatic radical, comprising the step of reacting a dihydroxyaromatic with a dihaloaromatic; wherein neither the dihydroxyaromatic nor the dihaloaromatic is present in an excess amount; and wherein the reaction is performed in the presence of a copper compound and cesium carbonate.
The invention further comprises a process of preparing the above polyaromatic ether comprising the step of reacting a halohydoxyaromatic in the presence of a copper compound and cesium carbonate.
The invention further comprises a process of preparing an aromatic ether oligomer comprising the formula: 
wherein Ar is an independently selected divalent aromatic radical; wherein T is a terminating group independently selected from the group consisting of xe2x80x94OH and xe2x80x94X; wherein X is independently selected from the group consisting of Br and I; and wherein n is an integer greater than or equal to 1; comprising the step of reacting a dihydroxyaromatic with a dihaloaromatic; wherein the reaction is performed in the presence of a copper compound and cesium carbonate; and wherein either the dihydroxyaromatic or the dihaloaromatic is present in an excess amount.
The invention further comprises a process of preparing a phthalonitrile monomer comprising the formula: 
wherein Ar is an independently selected divalent aromatic radical; and wherein n is an even integer greater than or equal to 2; comprising the step of reacting a 3- or 4-nitrophthalonitrile with a hydroxy-terminated aromatic ether oligomer.
The invention further comprises a process of preparing a thermoset comprising the step of curing a mixture comprising the above phthalonitrile monomer.
The invention further comprises the polyaromatic ether, aromatic ether oligomer, phthalonitrile monomer, and thermoset described above.
The synthesis of the thermoset is performed in three steps. First, a dihydroxyaromatic is reacted with a dihaloaromatic to form an aromatic ether oligomer. Second, the aromatic ether oligomer is reacted with a 3- or 4-nitrophthalonitrile to make a phthalonitrile monomer. Third, the phthalonitrile monomer is cured to make a thermoset. Any reference to an ingredient can refer to one embodiment of such ingredient or a combination of one or more embodiments. All polymeric and oligomeric structures claimed include all configurations, isomers, and tacticities of the polymers and oligomers within the scope of the claims. The term xe2x80x9coligomerxe2x80x9d as used herein does not place an any upper or lower limit on the chain length of the oligomer.
In the first step the dihydroxyaromatic is reacted with the dihaloaromatic to form the polyaromatic ether or the aromatic ether oligomer as shown in formula 1. 
The halo groups, X, on the dihaloaromatic can be iodo or bromo or a combination thereof. Each Ar is an independently selected divalent aromatic radical. The divalent aromatic radical can be any divalent radical with or without substituents containing one or more fused aromatic rings, one or more non-fused aromatic rings with or without intervening functional groups, or combinations thereof wherein the radical sites are on the same or different aromatic rings. 1,3-Phenylene and 1,4-phenylene are typical divalent aromatic radicals. The divalent aromatic radical can be different in each reactant. The divalent aromatic radical can also be different in multiple embodiments of the same reactant. For example, the dihydroxyaromatic can comprise a combination of any of resorcinol (m-dihydroxybenzene), hydroquinone (p-dihydroxybenzene), and any other dihydroxyaromatics. By further example, the dihaloaromatic can comprise a combination of any of m-dibromobenzene, p-dibromobenzene, m-diiodobenzene, p-diiodobenzene, m-bromoiodobenzene, p-bromoiodobenzene, and any other dihaloaromatics.
The aromatic ether oligomer or the polyaromatic ether has a structure that alternates between an aromatic ether functional group containing a divalent aromatic radical from the dihydroxyaromatic and an aromatic ether functional group containing a divalent aromatic radical from the dihaloaromatic.
In one embodiment neither the dihydroxyaromatic nor the dihaloaromatic is present in an excess amount, and the product is a polyaromatic ether. The polyaromatic ether can have a high molecular weight. Typically n is greater than or equal to 7. The polyaromatic ether is not necessarily convertible to a phthalonitrile monomer, but can be useful in other applications. Formula 2 shows the formation of a polyaromatic ether from a 1:1 molar ratio of hydroquinone and p-diiodobenzene. In another embodiment the polyaromatic ether is formed from a halohydroxyaromatic. Formula 3 shows the formation of a polyaromatic ether from 4-iodophenol. 
In another embodiment, either the dihydroxyaromatic or the dihaloaromatic is present in an excess amount to form an aromatic ether oligomer. This is shown in formula 4. 
The term n is an integer greater than or equal to 1. Typically, n is less than or equal to 100. More typically, n is equal to 2, 4, 6, or 8. T represents a terminating group. The terminating groups are independently selected from the group consisting of xe2x80x94OH or xe2x80x94X. In some embodiments, the same kind of terminating group is on both ends of the aromatic ether oligomer, although different embodiments of that kind may be found when the terminating group is xe2x80x94X. For example, when the dihaloaromatic is present in an excess amount and is 1-bromo-4-iodobenzene, both terminating groups can be xe2x80x94X, wherein any xe2x80x94X can be either xe2x80x94Br or xe2x80x94I. The process for making the aromatic ether oligomer with each terminating group is discussed separately.
When both terminating groups are xe2x80x94OH, the aromatic ether oligomer is a hydroxy-terminated aromatic ether oligomer. In this case, n is an even integer greater than or equal to 2. The hydroxy groups are bonded to the divalent aromatic radical from the dihydroxyaromatic. This structure is formed when the dihydroxyaromatic is present in an excess amount. When all the dihaloaromatic is consumed, there is still dihydroxyaromatic available to terminate the aromatic ether oligomer. Typically there is sufficient dihydroxyaromatic present to terminate both ends of all aromatic ether oligomeric molecules. If not, in some molecules one terminating group is xe2x80x94OH and the other is xe2x80x94X. Formula 5 shows the general reaction scheme and formula 6 shows the reaction of a 2:1 molar ratio of resorcinol and p-diiodobenzene. Formula 7 shows the reaction of a 2:1 molar ratio of resorcinol and 4,4xe2x80x2-diiodobiphenyl. 
The product in formula 6 represents the average length of the chain. The average length has three units, which corresponds to n=2. There may also be longer chain lengths present as well as unreacted resorcinol. Formula 7 illustrates a divalent aromatic radical with two non-fused aromatic rings. There can also be intervening functional groups between the aromatic rings, such as in bis(4-iodophenyl)methylene.
Formula 8 shows an example using a 3:2 ratio. The dihydroxyaromatic is resorcinol and the dihaloaromatic is a 1:1 molar combination of m-diiodobenzene and p-dibromobenzene. The average chain has five aromatic groups, which corresponds to n=4. Other configurations of the m-phenylene and p-phenylene groups from the dihaloaromatics can also be present as well as molecules with only m-phenylene or only p-phenylene groups from the dihaloaromatics. More than one dihydroxyaromatic can also be used either with a single dihaloaromatic or with more than one dihaloaromatic. Formula 8 shows a 1:1 molar ratio of two dihaloaromatics, however the molar ratios of more than one dihydroxyaromatics or dihaloaromatics can be any desired ratios. 
When both terminating groups are xe2x80x94X, the aromatic ether oligomer is a halo-terminated aromatic ether oligomer. In this case, n is an even integer greater than or equal to 2. The halo groups are bonded to the divalent aromatic radical from the dihaloaromatic. The halo-terminated aromatic ether oligomer is made when the dihaloaromatic is present in an excess amount. When all the dihydroxyaromatic is consumed, there is still dihaloaromatic available to terminate the aromatic ether oligomer. Typically there is sufficient dihaloaromatic present to terminate both ends of all aromatic ether oligomer molecules. If not, in some molecules one terminating group is xe2x80x94OH and the other is xe2x80x94X. The same variations of halo-terminated aromatic ether oligomers are possible as for hydroxy-terminated aromatic ether oligomers. Formula 9 shows the general reaction scheme. A 2:1 molar ratio of m-diiodobenzene and hydroquinone would react as in formula 10. The average chain has three aromatic groups, which corresponds to n=2. 
A second way to make a hydroxy-terminated aromatic ether oligomer is to react a halo-terminated aromatic ether oligomer with a dihydroxyaromatic. This dihydroxyaromatic can be the same or different from that used to make the halo-terminated aromatic ether oligomer. This process can be useful for making a hydroxy-terminated aromatic ether oligomer where the aromatic groups at the ends of the chain are different from those in the middle. The dihydroxyaromatic used in this step can also be a combination of dihydroxyaromatics. Formula 11 shows the general reaction scheme. Arxe2x80x3 is an independently selected divalent aromatic radical. Formula 12 shows the reaction of the product of formula 10 with 1,4-naphthalenediol. 1,4-naphthalenediol is an example of a dihydroxyaromatic having a divalent aromatic radical having two fused aromatic rings. 
A similar process can be used to form an aryl-terminated aromatic ether oligomer. This aromatic ether oligomer is made by reacting a hydroxy-terminated aromatic ether oligomer with a haloaromatic. The haloaromatic is a monovalent aromatic radical with either a bromo or iodo substituent. The monovalent aromatic radical can be any monovalent radical with or without substituents containing one or more fused aromatic rings, one or more non-fused aromatic rings with or without intervening functional groups, or combinations thereof wherein the radical site is on an aromatic ring. Phenyl is a typical monovalent aromatic radical. Typically, there is only one halo substituent. The haloaromatic can be a combination of haloaromatics. The haloaromatic reacts with the terminal hydroxide groups of the hydroxy-terminated aromatic ether oligomer to produce the aryl-terminated aromatic ether oligomer. Formula 13 shows the general reaction scheme. Formula 14 shows the reaction of the product of formula 6 with iodobenzene. 
The aryl-terminated aromatic ether oligomer can also be formed by reacting a halo-terminated aromatic ether oligomer with a hydroxyaromatic. The hydroxyaromatic is a monovalent aromatic radical with a hydroxy substituent. The same variations are possible as described in the previous paragraph.
All of the above reactions are performed in the presence of a copper compound and cesium carbonate. Typically the copper compound is CuI or CuBr. Other suitable copper compounds include, but are not limited to, CuCI, CuBr2, and CuSO4. Typically, the dihydroxyaromatic, dihaloaromatic, copper compound, and cesium carbonate are dissolved in solvent and heated. Typically, after the reaction is complete the aromatic ether oligomer can then be precipitated with an aqueous acidic solution. The average molecular weight of the aromatic ether oligomer or the polyaromatic ether is controlled by the ratio of the reactants as described above.
The hydroxy-terminated aromatic ether oligomers can be used to make the phthalonitrile monomers described below, as well as numerous new polymers and compounds through the reaction of the hydroxyl group.
In the second step, the hydroxy-terminated aromatic ether oligomer is reacted with 3- or 4-nitrophthalonitrile to make the phthalonitrile monomer. Neither a halo-terminated aromatic ether oligomer nor an aryl-terminated aromatic ether oligomer can be used in this step. Formula 15 shows the general reaction scheme. Formula 16 shows the reaction of the product of formula 6 with 4-nitrophthalonitrile. 
Typically, there is at least a 2:1 molar ratio of 3- or 4-nitrophthalonitrile to hydroxy-terminated aromatic ether oligomer to ensure that all terminal hydroxide groups react with the 3- or 4-nitrophthalonitrile. Any remaining unreacted terminal hydroxide groups can make it more difficult to control the reaction during the curing step. Typically, the hydroxy-terminated aromatic ether oligomer and the 3- or 4-nitrophthalonitrile are dissolved in a solvent and heated in the presence of a base.
The previous step of forming the hydroxy-terminated aromatic ether oligomer typically produces a combination of multiple hydroxy-terminated aromatic ether oligomers (including unreacted dihydroxyaromatic) having an average value of n. This combination can be reacted with the 3- or 4-nitrophthalonitrile to form a combination of phthalonitrile monomers having different values of n. This can result in some phthalonitrile monomers where n is zero.
In the final step, a mixture comprising the phthalonitrile monomer is cured to form the thermoset. The cyano groups are the cure sites. As these groups react with each other a cross-linked thermoset is formed. The mixture can comprise multiple phthalonitrile monomers having different values of n. Such a mixture is produced when the phthalonitrile monomers are produced from a combination of hydroxy-terminated aromatic ether oligomers having an average value of n.
The mixture can also comprise 4,4xe2x80x2-bis(3,4-dicyanophenoxy)biphenyl, bis[4-(3,4-dicyanophenoxy)phenyl]dimethylmethane, bis[4-(2,3-dicyanophenoxy)phenyl]dimethylmethane, bis[4-(3,4-dicyanophenoxy)phenyl]-bis(trifluoromethyl)methane, bis[4-(2,3-dicyanophenoxy)phenyl]-bis(trifluoromethyl)methane, 1,3-bis(3,4-dicyanophenoxy)benzene, or 1,4-bis(3,4-dicyanophenoxy)benzene. These compounds are also phthalonitrile monomers. The mixture can also comprise any compound with one or more phthalonitrile groups. Typically, these phthalonitrile compounds have two or more phthalonitrile groups. Such phthalonitrile compounds include, but are not limited to, the phthalonitrile monomers disclosed in the patents cited above. All these compounds can cure with the phthalonitrile monomers of the present invention.
Typically the mixture comprises a curing agent. The curing agent can be any substance useful in promoting the polymerization of the phthalonitrile monomer. More than one curing agent can be used. Typically, the same amount of curing agent can be used as conventionally used in curing analogous prior art monomers. Typically the curing agent is added to a melt of the phthalonitrile monomer with stirring. The mixture is then cured in one or more curing stages. Typical curing temperatures range from about 80xc2x0 C. to about 500xc2x0 C. More typically, the range is from 80xc2x0 C. to about 375xc2x0 C. Generally, more complete curing occurs at higher temperatures.
Suitable curing agents include, but are not limited to, aromatic amines, primary amines, secondary amines, diamines, polyamines, amine-substituted phosphazenes, phenols, strong acids, organic acids, strong organic acids, inorganic acids, metals, metallic salts, metallic salt hydrates, metallic compounds, halogen-containing aromatic amines, clays, and chemically modified clays. The use of clays or chemically modified clays may improve the mechanical and flammability properties of the thermoset. Typically, chemical modification of a clay involves replacing sodium ions with ammonium to form quarternary ammonium salts.
Specific curing agents include, but are not limited to, bis[4-(4-aminophenoxy)phenyl]sulfone (p-BAPS), bis[4-(3-aminophenoxy)phenyl] sulfone (m-BAPS), 1,4-bis(3-aminophenoxy)benzene (p-APB), 1,12-diaminododecane, diphenylamine, epoxy amine hardener, 1,6-hexanediamine, 1,3-phenylenediamine, 1,4-phenylenediamine, p-toluenesulfonic acid, cuprous iodide, cuprous bromide, 1,3-bis(3-aminophenoxy)benzene (m-APB), 3,3xe2x80x2-dimethyl-4,4xe2x80x2-diaminodiphenylsulfone, 3,3xe2x80x2-diethoxy-4,4xe2x80x2-diaminodiphenylsulfone, 3,3xe2x80x2-dicarboxy-4,4xe2x80x2-diaminodiphenylsulfone, 3,3xe2x80x2-dihydroxy-4,4xe2x80x2-diaminodiphenylsulfone, 3,3xe2x80x2-disulfo-4,4xe2x80x2-diaminodiphenylsulfone, 3,3xe2x80x2-diaminobenzophenone, 4,4xe2x80x2-diaminobenzophenone, 3,3xe2x80x2-dimethyl-4,4xe2x80x2-diaminobenzophenone, 3,3xe2x80x2-dimethoxy-4,4xe2x80x2-diaminobenzophenone, 3,3xe2x80x2-dicarboxy-4,4xe2x80x2-diaminobenzophenone, 3,3xe2x80x2-dihydroxy-4,4xe2x80x2-diaminobenzophenone, 3,3xe2x80x2-disulfo-4,4xe2x80x2-diaminobenzophenone, 4,4xe2x80x2-diaminodiphenyl ethyl phosphine oxide, 4,4xe2x80x2-diaminodiphenyl phenyl phosphine oxide, bis(3-aminophenoxy-4xe2x80x2-phenyl)phenyl phosphine oxide, methylene dianiline, hexakis(4-aminophenoxy)cyclotriphosphazene, 3,3xe2x80x2-dichloro-4,4xe2x80x2-diaminodiphenylsulfone, 2,2xe2x80x2-bis(trifluoromethyl)-4,4xe2x80x2-diaminobiphenyl, 2,2xe2x80x2-bis(4-aminophenyl)hexafluoropropane, bis[4-(4-aminophenoxy)phenyl]2,2xe2x80x2-hexafluoropropane, 1,1-bis(4-aminophenyl)-1-phenyl-2,2,2-trifluoroethane, 3,3xe2x80x2-dichloro-4,4xe2x80x2-diaminobenzophenone, 3,3xe2x80x2-dibromo-4,4xe2x80x2-diaminobenzophenone, aniline-2-sulfonic acid, 8-aniline-1-naphthalenesulfonic acid, benzene sulfonic acid, butylsulfonic acid, 10-camphorsulfonic acid, 2,5-diaminobenzenesulfonic acid, 6-dimethylamino-4-hydroxy-2-naphthalenesulfonic acid, 5-dimethylamino-1-naphthalenesulfonic acid, 4-hydroxy-3-nitroso-1-naphthalenesulfonic acid tetrahydrate, 8-hydroxyquinoline-5-sulfonic acid, methylsulfonic acid, phenylboric acid, 1-naphthalenesulfonic acid, 2-naphthalenesulfonic acid, 1,5-naphthalenedisulfonic acid, 2,6-naphthalenedisulfonic acid, 2,7-naphthalenedisulfonic acid, picrylsulfonic acid hydrate, 2-pyridineethanesulfonic acid, 4-pyridineethanesulfonic acid, 3-pyridinesulfonic acid, 2-pyridinylhydroxymethanesulfonic acid, sulfanilic acid, 2-sulfobenzoic acid hydrate, 5-sulfosalicylic acid hydrate, 2,4-xylenesulfonic acid, sulfonic acid containing dyes, organic phosphorus-containing acids, phenylphosphinic acid, diphenylphosphinic acid, propylphosphonic acid, 1-aminoethylphosphonic acid, 4-aminophenylphosponic acid, butylphosphonic acid, t-butylphosphonic acid, 2-carboxyethylphosphonic acid, 2-chloroethylphosphonic acid, dimethylphosphonic acid, ethylphosphonic acid, methylenediphosphonic acid, methylphosphonic acid, phosphonoacetic acid, bis(hydroxymethyl)phosphonic acid, chloromethylphosphonic acid, di-n-butylphosphonic acid, dichloromethylphosphonic acid, diphenyldithiophosphonic acid, 1,2-ethylenediphosphonic acid, n-hystaderylphosphonic acid, hydroxymethylphosphonic acid, n-octadecylphosphonic acid, n-octylphosphonic acid, phenylphosphonic acid, propylenediphosphonic acid; n-tetradecylphosphonic acid, concentrated sulfuric acid, phenylphosphonic acid, copper, iron, zinc, nickel, chromium, molybdenum, vanadium, beryllium, silver, mercury, tin, lead, antimony, calcium, barium, manganese, magnesium, cobalt, palladium, platinum, stannous chloride, cuprous bromide, cuprous cyanide, cuprous ferricyanide, zinc chloride, zinc bromide, zinc iodide, zinc cyanide, zinc ferrocyanide, zinc acetate, zinc sulfide, silver chloride, ferrous chloride ferric chloride, ferrous ferricyanide, ferrous chloroplatinate, ferrous fluoride, ferrous sulfate, cobaltous chloride, cobaltic sulfate, cobaltous cyanide, nickel chloride, nickel cyanide, nickel sulfate, nickel carbonate, stannic chloride, stannous chloride hydrates, stannous chloride dihydrate, aluminum nitrate hydrates, aluminum nitrate nonahydrate, triphenylphosphine oxide complex, montmorillonite, and chemically modified montmorillonite.
The invention has the advantage of using a low melting phthalonitrile monomer. As the value of n increases, the processing temperature of the phthalonitrile monomer is shifted to lower temperatures. The low melting point allows the monomer to have a lower viscosity at a given temperature than other phthalonitrile monomers. A low viscosity resin enables composite processing by resin transfer molding, resin infusion methods, and filament winding, without heating the curing mixture to a temperature that initiates curing. Curing can be initiated when the mixture is in position and need not flow any further. Furthermore, a low melt viscosity and a larger processing window are useful for fabrication of thick composite sections where the melt must impregnate thick fiber preforms. The curing mixture viscosity is a function of both the curing agent concentration and the melt temperature. Thus, low melting phthalonitrile monomers and curing agents that do not volatilize at elevated cure temperatures can enhance the processability of phthalonitrile-based composites. This is important since most high temperature resins are not amenable to processing by cost effective methods such as resin transfer molding, resin infusion molding, filament winding, and oven cure due to high initial viscosities, the evolution of volatiles during the cure, and solvent-related problems.
The thermoset has the advantage of very desirable thermo-oxidative properties, which may be unaffected by the nature of the curing agent. The thermoset also has improved physical properties, such as toughness and processability, relative to systems with a short spacer between the terminal phthalonitrile moieties. Generally, toughness and brittleness are improved with lower cross-link densities. This can be achieved by using phthalonitrile monomers with longer spacer chains.